When it comes to precipitation type, Winter in Southern New England has all the bases covered. We get it all... rain, snow, sleet, freezing rain and sometimes a combination. But a lot of people don't know the difference between freezing rain and sleet (a lot of people don't care, either, but I digress). Let's start with freezing rain. It is nothing more than rain that falls into a shallow layer of sub-freezing air near the ground and hence freezes on contact with objects at the surface, i.e. car windows, lamp posts, power lines, even roads. As the rain continues, the ice continues to build up on these objects. Some ice storms can actually produce an inch of ice, or more. This causes deadly driving conditions and can also cause power outages due to ice-covered tree branches breaking and falling onto power lines. Again, freezing rain is just rain that happens to be falling onto surfaces that are below freezing.
That brings us to sleet, which is also known as ice pellets. Sleet occurs when there is a very thin layer of warm air at mid-levels of a storm. When the snow falls into that thin mid-level warm air, it melts and changes to rain. But as the rain continues its descent, it then encounters a rather thick lower layer of sub-freezing air. If that lower cold layer is thick enough (usually several thousand feet), the rain has enough time to re-freeze into an ice pellets, or sleet.
In summary, the determining factor in whether precipitation is freezing rain or sleet is the depth of the warm layer at mid levels and the depth of the cold layer extending upwards from the ground. Freezing rain (sleet) occurs when the mid level warm layer is relatively thick (thin), and the cold layer near the ground is relatively shallow (thick).